1. Field of the Invention
The present invention relates to method for producing a halftone phase shift mask blank, a halftone phase shift mask blank produced by the method thereof, and a halftone phase shift mask.
2. Description of the Related Art
The recent photolithography requires two important factors, namely, high resolution and a focus depth, which, however, are contradictory to each other. It has been found out that practical resolution in photolithography can not be increased even if the numerical aperture (NA) of lenses in exposure devices is enlarged, or the wavelength of the light that the lenses therein shall receive is shortened (see, for example, Monthly Semiconductor World, December 1990; Applied Physics, Vol. 60, November 1991).
Under such a circumstance, phase shift lithography is promising as a next-generation technology in the art of photolithography. The phase shift lithography increases the resolution in photolithography only by shifting the phase of photomasks with no change in optical systems. In such phase shift lithography, concretely, the light running through a photomask undergoes phase shift to thereby remarkably increase the resolution based on the mutual interference of the transmitted light.
The phase shift mask in that technology possesses both light intensity information and phase information, and various types of the mask are known, including, for example, Levenson masks, pattern-assist masks, self-alignment masks (edge-stress masks), etc. Compared with conventional photomasks possessing light intensity information only, these phase shift masks have a complicated structure and require a high-level technique for their production.
Recently, a halftone phase shift mask as one type of such phase shift masks has been developed. In its semi-transmission area, the halftone phase shift mask possesses two functions; one is for substantially shielding the light applied thereto, and the other is for shifting the light phase (in general, for phase inversion). Therefore, the mask of the type does not require a shielding film pattern and a phase shift film pattern separately formed thereon, and its advantage is that its constitution is simple and its production is easy.
As illustrated in FIGS. 1A through 1C, the mask pattern formed on a transparent substrate 100 comprises a transmission area 200 (in which the substrate is exposed out) that transmits the light to substantially participate in pattern transfer, and a semi-transmission area 300 (which acts both for light shielding and for phase shifting) that transmits the light substantially not to participate in pattern transfer (FIG. 1A). The phase of the light having passed through the semi-transmission area is so shifted that the thus-shifted light phase may be substantially the inverse of the phase of the light having passed through the transmission area (FIG. 1B). Thus, the light having passed through the part around the boundary between the semi-transmission area and the transmission area and having entered the opposite areas through diffraction can be mutually canceled out so that the light intensity at the boundary may be nearly zero and the contrast at the boundary is increased, or that is, the resolution for pattern transfer is thereby increased (FIG. 1C).
The semi-transmission area of the halftone phase shift mask of the type mentioned above must satisfy the optimum requirements of both the light transmittance and the phase shift through it.
For most suitably controlling the necessary optimum requirements of the mask, one method has been proposed, which comprises forming an alternate laminate of silicon nitride and titanium nitride for the semi-transmission area of the mask (L. Dieu, P. F. Carcia, H. Mitsui, and K. Ueno, “Ion Beam Sputter-Deposited SiN/TiN Attenuating Phase-Shift Phtoblanks”, SPIE, 4186, 810 (2000)).
The semi-transmission area of the phase shift mask blank proposed has a multi-layered structure of an alternate laminate made of silicon nitride having high transmissibility for ArF excimer laser (193 nm) and titanium nitride of good shieldability from the laser, therefore realizing both the transmittance and the phase angle suitable to phase shift masks.
For controlling the transmittance of the mask from the blank, the overall film ratio of the silicon nitride layers to the titanium nitride layers is varied.
The number of the silicon nitride and titanium nitride layers that constitute the laminate structure of the mask is so controlled that the thickness of one layer is at most {fraction (1/10)} of the wavelength of the light to be applied to the mask. In that condition, the multi-layered semi-transmission area of the mask may be optically considered as a single layer for exposure to light.
Another advantage of the multi-layered phase shift mask in which the thickness of each layer is reduced is that the profile of the pattern cross-section is prevented from being deformed.
Phase shift mask blanks are required to be resistant to chemicals such as acid and alkali solutions in order that the phase angle and the transmittance of the masks from them do not vary while they are indispensably washed with such solutions in the process of processing them into masks. Silicon nitride is highly resistant to chemicals such as acid and alkali solutions, and mask blanks of which the surface layer of the semi-transmission area is made of silicon nitride may have good chemical resistance.
In addition to the problems with the recent phase shift masks noted above that are caused by the material of the semi-transmission film of the masks, other serious problems with them are particles often adhering to the mask blanks and pin holes often formed in the masks. For example, in a conventional magnetron sputtering device for forming a semi-transmission film on mask blanks, particles having a size of about 1 micron or smaller may often adhere to the film formed, owing to the arcs generated in the device. The particles thus having adhered to the film are dropped off while the processed masks are washed, thereby to form pin holes in the masks. For producing masks having few pin holes, therefore, it is necessary to reduce the number of the particles that may adhere to the semi-transmission film while the film is formed.
For reducing the particles to adhere to the semi-transmission film, the arcs in the process of film formation must be controlled. However, a titanium target for forming a film of titanium nitride easily reacts with nitrogen, and titanium nitride to cause arcs readily deposits on the target. Therefore, in principle, ion beam sputtering that causes few arcs is desired for forming a titanium nitride film with few particles adhering thereto. In the device of ion beam sputtering of a target onto a substrate, ion generation and acceleration necessary for the target sputtering are effected in an ion source unit disposed separately from the target and the substrate, and therefore, high voltage application to the target that causes arcs is unnecessary.
In addition, in the ion beam sputtering device, the gas pressure between the target and the substrate is low, as compared with that in the magnetron sputtering device. In the ion beam sputtering device, therefore, the particles sputtered from the target may directly enter the substrate, not colliding against gas molecules, and, in addition, their kinetic energy is large. Accordingly, in this, dense and smooth films of low surface roughness can be formed.
If dense and smooth films of silicon nitride and titanium nitride are alternately formed through such ion beam sputtering, it is possible to prevent the mutual diffusion in the interface between the neighboring films in the alternate multi-layered structure while the laminate structure is exposed to light, and the laminate structure is highly durable to exposure to light.
However, the ion beam sputtering process of forming a silicon nitride film still has the following problems.
Specifically, in the process, since the gas pressure between the target and the substrate is low and the kinetic energy of the particles sputtered from the target is high, the silicon nitride film formed often receives compression stress (T. Carriere, B. Agius, I. Vickridge, J, Siejko, P. Alnot, “Characterization of Silicon Nitride Films Deposited on GaAS by RF Magnetron Cathodic Sputtering”, J. Electrochem. Soc., Vol. 137, No. 5, 1582 (1990); and H. Windischmann, “An intrinsic stress scaling law for polycrystalline thin films prepared by ion beam sputtering”, J. Appl. Phys., 62(5), 1800 (1987)). The compression stress of the silicon nitride film is kept still as such even when it is formed in a multi-layered semi-transmission film structure. In that condition, the internal stress of the multi-layered semi-transmission film worsens the planarity of the transparent substrate having the film formed thereon, and reduces the depth of focus of the film exposed to light. Still another problem is that, if the internal stress of the semi-transmission film is large, the substrate deforms before and after the patterning process, and therefore worsens the dimensional accuracy in patterning thereon.